Most brokers who tailor to individual investors have dropped the commissions and transaction fees to zero (e.g. Fidelity, (Links to an external site.) RobynHood (Links to an external site.))

However, all of the brokers who tailor to professional investors/traders still charge a fee, typically on a 'per share' basis (e.g. $0.005 per share). The biggest of them is Interactive Brokers. (Links to an external site.)

A typical professional trader might buy 20,000 shares and quickly (after a few minutes) sell the same shares for a quick profit. She could pay ZERO for the two transactions ("buy" and then "sell") if she used TD Ameritrade or 20,000 x 0.005 x 2=$200 if she used Interactive Brokers. Huge difference if your only goal is to make more money!

Question: Why would she (and most other professional traders) still prefer Interactive Brokers even for very large orders (like a million shares, where the transaction fee would be $10,000)? Offer some plausible explanations and reasoning for their seemingly irrational behavior